1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, to an organic electroluminescence (EL) device.
2. Discussion of the Related Art
Among flat panel displays, an organic electroluminescence (EL) device is a self-emission type display with higher contrast and wider viewing angle as compared to a liquid crystal display (LCD). The organic EL device can be made lightweight and slim profile as compared to other display types because it does not require a backlight. The organic EL device also uses less power than other types of flat panel displays. Further, the organic EL device can be driven with a low DC voltage and still have a rapid response rate. Since all of the components of the organic EL device are formed of solid materials, it can withstand an impact. The organic EL device can operate throughout a wide temperature range and be manufactured at a low cost. Unlike fabricating an LCD or a PDP, the organic EL device is manufactured just using a deposition process and an encapsulation process. Thus, the manufacturing processes and apparatuses for making an organic EL device are very simple.
A passive matrix type organic EL device without a switching element has been widely used. In the passive matrix type, gate lines (scan lines) cross data lines (signal lines) to define a matrix of sub-pixels. The gate lines are sequentially driven to drive each sub-pixel. To exhibit a required mean luminescence, a higher level of moment luminance must be emitted sequentially in each sub-pixel across the display to create an overall average luminance.
In an active matrix type, thin film transistors acting as switching elements are located in respective sub-pixels. The voltage applied to the sub-pixels are charged in a storage capacitor Cst so that the voltage can be applied until a next frame signal is applied, thereby continuously driving the organic EL device, regardless of the number of gate lines, to display a picture. Accordingly, in the active matrix type, even when low current is applied, uniform luminescence can be obtained. As a result, the organic EL device has the advantages of low power consumption, high definition and large-sized screen capability. Such an active matrix type organic EL device will now be described with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view schematically showing a bottom emission organic EL device according to the related art, wherein a unit pixel region includes red (R), green (G) and blue (B) sub-pixels. As shown in FIG. 1, the related art bottom emission organic EL device includes a first substrate 10 and a second substrate 30 facing each other and sealed by a seal pattern 40. The first substrate 10 includes a transparent substrate 1, thin film transistors T formed in each sub-pixel on the transparent substrate 1, first electrodes 12 connected with the thin film transistors T, an organic EL layer 14 connected with the thin film transistors T and disposed corresponding to the first electrodes 12 on the thin film transistors T, and a second electrode 16 formed on the organic EL layer 14. The organic EL layer 14 includes emission materials emitting red (R), green (G) and blue colors. The first and second electrodes 12 and 16 apply an electric field to the organic EL layer 14.
The second electrode 16 is spaced away from the second substrate 30 by the seal pattern 40. A moisture absorbent (not shown) for preventing moisture from leaking to an outside is filled into an inner surface of the second substrate 30 and fixed by a semi-transparent tape (not shown). In the related art bottom emission structure, the first electrode 12 serves as an anode and is selected from a group consisting of transparent conductive materials, whereas the second electrode 16 serves as a cathode and is selected from a group consisting of metal materials having a low work function. Thus, the organic EL layer 14 has a stack structure where a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, an electron transporting layer 14d that are sequentially stacked starting from the hole injection layer 14a contacting the first electrode 12. Herein, the emission layer 14c has a structure in which the emission materials emitting red (R), green (G) and blue colors are sequentially arranged corresponding to the respective sub-pixels.
The related art organic EL device has a limitation in fine-patterning the red (R), green (G) and blue pixels with high reproduction under a large area. For example, since the organic EL material for the organic EL layer 14 is vulnerable to solvent or moisture, it cannot be patterned by a wet etch. For this reason, the organic EL material cannot be patterned by photolithography, which is advantageous in forming fine patterns.
Low molecular organic EL material can be patterned by a method including installing a fine-patterned shadow mask on a substrate and then independently forming R, G, B materials. However, this method is limited in precisely fabricating the shadow mask to have fine patterns over a resolution of a predetermined level and employing the shadow mask in a high definition and large area due to the tension deviation of the shadow mask and the like. Also, another pixel patterning method using a high molecular organic EL material inkjet injection head has been researched, but it is difficult for forming a pinhole-free thin film less than 1000 Å.
The related art bottom emission structure organic EL devices are fabricated by attaching the first substrate 10 provided with an array device and an organic EL diode to the second substrate 30 for separate encapsulation. Thus, a yield of the organic EL display is determined by both of a yield of the array device and a yield of the organic EL diode, and therefore, an overall process yield is greatly limited to a latter process, namely, the process of forming the organic EL diode. For example, even though the array device is formed excellently, if defects occur due to foreign matters or other factors in forming the organic EL layer employing a thin film of about 1000 Å thick, the entire organic EL device is rendered defective. Consequently, a defective organic EL layer results in decreased production yield and increased material costs in manufacturing the non-defective array element associated with the defective organic EL layer.
In addition, since the bottom emission structure has high stability and high process freedom because of the encapsulation, but has a limitation in the aperture ratio, it is suitable to employ the bottom emission structure for high resolution products.